I previously stayed at Emerald Guesthouse for two nights, around 11 days before this second stay. Very easy to sign up for the shuttle when you check in, to take you from Emerald Guest to either the nearest train station (fast train takes you to Hop on Hop Off bus ticket office) or other places. or of course the airport which is about five minutes at most in the van. To catch the bus from the airport you just walk to the bus station part of the airport which is about a two minute walk from International Arrivals (try to avoid the porters or the people at the information counter there, who will ask you for money for lunch if you stand around there too long). Shuttle van leaves the airport bus stop at the start of each hour so you don't have to annoyingly phone the driver or anything, you just approach him when the van arrives. You'll quickly work out which guys work in the bus stop and are telling the buses, vans and shuttles where to park, so if you've arrived just after the hour ask one of them if it has come yet. They do know.
You also get free breakfast which you pick from a menu what you want included when you check in. It's not huge but it is good. The place is pretty basic looking, but it is all you need and was very cheap when I booked. Complex is fenced for our safety.
Kitchen staff, check in staff and the free shuttle driver are all really friendly. Wouldn't hesitate to book to stay here again next time visit Johannesburg.